What is cross-training good for and what won't it help? The short answers are easy: it's good for health, but not necessarily good for performance. The longer answers, as explored by Gina Kolata in the New York Times, are not so easy. And not so definite. Kolata cites research by Hirofumi Tanaka, an exercise physiologist at the University of Texas in Austin, whose review of published papers comparing athletes, both trained and untrained, found that only one factor mattered if performance was the goal: training in that sport. She also mentions a more recent study involving moderately fit runners and the other trained runners that found that adding cycling to a running program did not improve running performance. Things get confusing, says Kolata, when you throw weight training into the mix. In a review of published studies, Tanaka found that resistance training improved endurance in running and cycling for both in experienced athletes and novices. Another study found that runners who did half-squats with heavy weights three times a week improved their running efficiency and endurance. The same regimen also helped cyclists, Kolata says, but did nothing for swimmers, although Tanaka found that swimmers do get faster with a specific type of resistance training, done while in the water, that concentrates on the movements they use in their strokes.
Read more about the benefits of cross-training in the New York Times.
Apart from objective measures of performance (which don’t matter much to the less competitive among us), the advantage of cross training is staying fresh, having fun. Most people eventually burn out on a sport if it’s all they do. Doing something, anything, different that you just plain like (no matter what level you perform at) makes your “primary” sport(s) more resistant to negative issues.